Abstract
We report a comprehensive study of turbulent superfluid He-4 flow through a channel of square cross section. We study for the first time two distinct flow configurations with the same apparatus: coflow (normal and superfluid components move in the same direction), and counterflow (normal and superfluid components move in opposite directions).
We realise also a variation of counterflow with the same relative velocity, but where the superfluid component moves while there is no net flow of the normal component through the channel, i.e., pure superflow. We use the second-sound attenuation technique to measure the density of quantised vortex lines in the temperature range 1.2 K less than or similar to T less than or similar to T-lambda approximate to 2.18 K and for flow velocities from about 1 mm/s up to almost 1 m/s in fully developed turbulence.
We find that both the steady-state and temporal decay of the turbulence significantly differ in the three flow configurations, yielding an interesting insight into two-fluid hydrodynamics. In both pure superflow and counterflow, the same scaling of vortex line density with counterflow velocity is observed, L proportional to V-cf(2), with a pronounced temperature dependence; in coflow instead, the vortex line density scales with velocity as L proportional to V-3/2 and is temperature independent; we provide theoretical explanations for these observations.
Further, we develop a new promising technique to use different second-sound resonant modes to probe the spatial distribution of quantised vortices in the direction perpendicular to the flow. Preliminary measurements indicate that coflow is less homogeneous than counterflow/superflow, with a denser concentration of vortices between the centre of the channel and its walls. (C) 2015 AIP Publishing LLC.